Induction heating may be used to preheat metal before welding. It is well known to weld large and relatively thick pieces of steel (or other material) together. For example, pipes are often formed by taking a flat piece of steel and rolling the steel. A longitudinal weld is then made along the ends of the rolled steel, thus forming a section of pipe. A pipe line may be formed by circumferential welding adjacent sections of pipe together. Other applications of welding relatively thick steel (or other material) include ship building, railroad yards, tanker trucks, or other higher strength alloy welding.
When welding such relatively thick steel (or other material) it is generally desirable to preheat the workpiece along the weld path. Pre-heating is used to raise the temperature of the workpiece along the weld path because the filler metal binds to the workpiece better when the weld path is heated, particularly when a high-alloy steel is being welded. Without preheating there is a greater likelihood that the filler metal won't properly bind with the workpiece, and a crack may form. Generally, the steel is preheated to about 300° F. prior to welding. Preheating is often used for heating steel having a thickness of more than about ½″.
Generally, in the prior art, “rose buds” (gas fired flame torches), resistance “chicklets” or induction heating blankets are used to preheat the steel. The rosebuds are placed along the weld path, typically one rosebud on each side of the weld path, or one covering both sides of the weld path, every 3 to 6 feet. The rosebuds are left in place a relatively long period of time (for example up to two hours for 3″ thick steel). After the weld path has been preheated, the rose buds are removed and the weld is performed before the weld path cools.
Induction heating blankets are used to preheat a weld by wrapping an induction blanket (an induction cable inside a thermally safe material), and inducing current in the workpiece. Induction heating can be a fast and reliable way to preheat, particularly on stationary workpieces. However, induction blankets are not particularly useful on moving workpieces, and some pipe welding applications have a fixed position welder with a pipe that moves or rotates past the weld location. Liquid cooled cables offer flexibility in coil configuration, but have similar issues with rotating pipes rolling up cables or wearing through the insulation.
Other methods of preheating a weld path include placing the entire workpiece in an oven (which takes as long as using a rosebud), induction heating, or resistance heating wires. When preheating with these alternatives in the prior art, the heating device is placed at one location on the weld path until that location is heated. Then the weld is performed and the heating device is moved.
A significant advance in the prior art was a system that can preheat “on the fly” just in advance of the weld. Such a system is described in U.S. Pat. No. 6,265,701 (hereby incorporated by reference), and is owned by the owner of this application. Prior art induction coils had a puck design (flat, single turn) or a helical design with one turn in each plane. Such a system requires that the coil match the output impedance of the power source for efficient induction heating. As described in U.S. Pat. No. 6,265,701, impedance matching was accomplished therein by changing capacitors inside the power source—i.e., the power source is tuned to the coil. It is not always desirable to have the power source components readily accessible in the field. Another solution has been to insert a matching transformer between the power source output and the induction head. The transformer steps down the output voltage to match impedances. However, these matching transformers can be expensive and result in a lower applied voltage to the coil.
Additionally, if preheating is done on the fly, and the workpiece is moving, there must be some way to hold the coil in the proper location. Some end users have made fixtures to hold coils in place relative to the desired heat zone. However, such fixtures can cause wear to the coils, do not provide a consistent coil-heat zone distance, they require matching transformers, they have inadequate coupling between the coil and the heat zone, they typically have a narrow heat zone that can overheat and discolor the surface of the material being heated, they do not work well with smaller diameter pipes, such as 8 inches, because the curve of the pipe results in decoupling at the edge of the heat zone, thereby reducing efficiency, they often use temperature sensitive crayons so the user can monitor the temperature but do not have temperature feedback for controlling the power source, and they are not typically expandable.
Accordingly, a system for preheating weld path is desirable. Preferably, such a system will be useful as a handheld system and/or be useful with a moving workpiece and stationary induction head. Also, such a system preferably does not require a matching transformer, or require the power source to be tuned to the coil, rather the coil is tuned to the power source. Also, such a system preferably maintains a constant coil-to-heat zone distance, and/or includes temperature feedback, and/or is expandable.